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11/5/2011

UNIT 10C Concurrency: Multitasking &

15110 Principles of , Carnegie 1 Mellon University - CORTINA

Multitasking & Operating Systems

• Multitasking - The coordination of several computational processes on one . • An (OS) is the system software responsible for the direct control and management of hardware and basic system operations. • An OS provides a foundation upon which to run application software such as word processing programs, web browsers, etc. from Wikipedia

15110 Principles of Computing, Carnegie 2 Mellon University - CORTINA

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Operating System “Flavors”

• – System , BSD, – Proprietary: Solaris, Mac OS X • Windows – Windows XP – Windows Vista – Windows 7

15110 Principles of Computing, Carnegie 3 Mellon University - CORTINA

Multitasking

• Each (un-blocked) application runs for a very short time on the computer's processor and then another runs, then another... • This is done at such a fast rate that it appears to the computer user that all applications are running at the same time. – How do actors appear to move in a motion picture?

15110 Principles of Computing, Carnegie 4 Mellon University - CORTINA

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Critical Section

• A critical section is a section of computer code that must only be executed by one process or at a time. • Examples: – A printer queue receiving a file to be printed – Code to set a seat as reserved – Web that generates and returns a web page showing current registration in course

15110 Principles of Computing, Carnegie 5 Mellon University - CORTINA

A Critical Section

Cars may not make turns through this intersection. They may only proceed straight ahead. When a car stops at this intersection, it can proceed straight ahead if there is no car to its left and no car to its right. Otherwise it must some random amount of seconds and then check again to see if it's safe to proceed. If not, it waits again, and so on.

15110 Principles of Computing, Carnegie 6 Mellon University - CORTINA

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Shared Computing Resources

• memory • tape drives • disk drives • printers • communication ports • input devices (keyboard, mouse)

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Deadlock

• Deadlock is the condition when two or more processes are all waiting for some shared resource that other processes of the group hold, causing all processes to wait forever without proceeding. • How can deadlock occur at the intersection with the 4-way stop?

15110 Principles of Computing, Carnegie 8 Mellon University - CORTINA

4 11/5/2011

Dining Philosopher’s Problem

Socrates

Aristotle Plato

Homer

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The Dining Philosopher’s

• Each philosopher thinks for a while, then picks up his left , then picks up his right fork, then eats, then puts down his left fork, then puts down his right fork, thinks for a while... – We assume here that each philosopher thinks and eats for random times, and a philosopher cannot be interrupted while he picks up or puts down a single fork. • Each fork models a "resource" on a computer controlled by an OS. • Original problem proposed by Edsgar Dijkstra.

15110 Principles of Computing, Carnegie 10 Mellon University - CORTINA

5 11/5/2011

Dining Philosopher’s Problem

• There are N philosophers. • Philosopher i does the following: 1. THINK N=4

2. Pick up fork i. Fork 3 Fork 2 3. Pick up fork (i+1) modulo N. 2 4. EAT 3 1 5. Put down fork i. 0 6. Put down fork (i+1) modulo N. Fork 0 Fork 1 7. Go to step 1. NOTE: (i+1) modulo N = i+1 , if 0 < i < N-1 (i+1) modulo N = 0, if i = N-1

15110 Principles of Computing, Carnegie 11 Mellon University - CORTINA

Dining Philosopher’s Problem

• There are N philosophers. • Philosopher i does the following: 1. THINK N=4

2. Pick up fork i. Fork 3 Fork 2 3. Pick up fork (i+1) modulo N. 2 4. EAT 3 1 5. Put down fork i. 0 6. Put down fork (i+1) modulo N. Fork 0 Fork 1 7. Go to step 1. How can deadlock occur here?

15110 Principles of Computing, Carnegie 12 Mellon University - CORTINA

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Removing Deadlock • Philosopher i performs the following: 1. THINK 2. If i is not equal to N-1: a. Pick up fork i b . Pick up fork i+1 N=4 3. If i equals N-1: a. Pick up fork 0 Fork 3 Fork 2 b . Pick up fork N-1 2 4. EAT 5. If i is not equal to N-1: 3 1 a. Put down fork i 0 b . Put down fork i+1 Fork 0 Fork 1 6. If i equals N-1: a. Put down fork 0 b . Put down fork N-1 Can a philosopher still starve? 7. Goto step 1 15110 Principles of Computing, Carnegie 13 Mellon University - CORTINA

Semaphores

• A (binary) S is a shared variable that holds an integer that is initialized to 1. • We can use a semaphore to protect a critical section so that only one process accesses this section at a given time: Initially, semaphore S is 1. The non-critical program code first process to request S sets S to 0, other processes Request S from proceeding once they get to their request operations. Once CRITICAL SECTION the first process finishes running the critical section, it releases S, Release S causing S to increase to 1, non-critical program code allowing another process to proceed into the critical section.

15110 Principles of Computing, Carnegie 14 Mellon University - CORTINA

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Summary

• and multitasking are very challenging problems in computer science. • The simple rules of sequential computation no longer apply when we compute – in a distributed manner across networks – in a single computer system in a multitasking environment

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